Only $35.99/year

Biology 3201 Unit (Long Answer)

Terms in this set (18)

Simplified Stages of Mitosis and Cytokinesis:

Prophase:

During the prophase stage of mitosis the duplicated DNA condenses into compact structures known as chromosomes. Remember the DNA has already been duplicated. While the DNA is condensing into chromosomes, the nuclear envelope surrounding the DNA begins to break down. This happens so the DNA compacted into these chromosome structures can be accessed by the rest of the cell.

Prophase of Mitosis

Metaphase:

In the metaphase stage of mitosis the chromosomes align in the center of the cell called the equatorial plate (like the equator of the Earth). Organelles known as centrioles move to the polar ends of the cell and project thin spindle fibers to connect to the center (centromeres) of each chromosome. Essentially the cell is grabbing each chromosomes at its center so it can pull it into equal pieces.

metaphase of mitosis

Anaphase:

While in the anaphase stage of mitosis the centrioles will begin to pull each chromosome into two halves called sister chromatids. Each chromatid contains the same information.

anaphase of mitosisTelophase:

Finally in the Telophase stage of mitosis the nuclear membrane forms around the chromatids and they are completely located at opposite ends of the cell.

telophase of mitosisCytokinesis:

Usually after Telophase the cell will also divide its cytoplasm and pinch off into two separate but identical daughter cells. Each daughter cell is an exact copy of the parent cell before the DNA was duplicated during Interphase.

cytokinesis after mitosis
Step One
MEIOSIS I: This is basically like the PMATI of a regular mitosis. Pairs of chromosomes are lined up at the center of the cell and then pulled to each side. Meiosis is a bit different because there something called crossing-over happens with the DNA.

This crossing over is an exchange of genes. The genes are mixed up, not resulting in a perfect duplicate like mitosis. The cell divides, leaving two new cells with a pair of chromosomes each. Normally the cell would begin to go about its business of living and slowly duplicate the chromosomes for another mitotic division. Since this is meiosis, there is a very short interphase and division begins again.

Step Two
MEIOSIS II: In Prophase II the DNA that remains in the cell begins to condense and form short chromosomes. Each chromosome pair has a centromere. The centrioles also begin their journey to opposite sides of the cell. In Metaphase II all of the chromosomes line up along the center of the cell and the centrioles are in position for the duplication. Anaphase II shows the chromosomes split and move to opposite sides of the cell. Each one splits into two pieces. They don't divide up the DNA between the new cells; they split the DNA that exists. Each daughter cell will get one-half of the DNA needed to make a functioning cell.

Telophase II shows the DNA completely pulled to the sides and the cell membrane begins to pinch. When it's all over, you are left with four haploid cells that are called gametes. The eventual purpose of the gametes will be to find other gametes with which they can combine. When they do, they will form a new organism.
Spermatogenesis
The male testes have tiny tubules containing diploid cells called spermatogonium that mature to become sperm. The basic function of spermatogenesis is to turn each one of the diploid spermatogonium into four haploid sperm cells. This quadrupling is accomplished through the meiotic cell division detailed in the last section. During interphase before meiosis I, the spermatogonium's 46 single chromosomes are replicated to form 46 pairs of sister chromatids, which then exchange genetic material through synapsis before the first meiotic division. In meiosis II, the two daughter cells go through a second division to yield four cells containing a unique set of 23 single chromosomes that ultimately mature into four sperm cells. Starting at puberty, a male will produce literally millions of sperm every single day for the rest of his life.

Oogenesis
Just like spermatogenesis, oogenesis involves the formation of haploid cells from an original diploid cell, called a primary oocyte, through meiosis. The female ovaries contain the primary oocytes. There are two major differences between the male and female production of gametes. First of all, oogenesis only leads to the production of one final ovum, or egg cell, from each primary oocyte (in contrast to the four sperm that are generated from every spermatogonium). Of the four daughter cells that are produced when the primary oocyte divides meiotically, three come out much smaller than the fourth. These smaller cells, called polar bodies, eventually disintegrate, leaving only the larger ovum as the final product of oogenesis. The production of one egg cell via oogenesis normally occurs only once a month, from puberty to menopause.

Flickr Creative Commons Images

Some images used in this set are licensed under the Creative Commons through Flickr.com.
Click to see the original works with their full license.